The Forbes 400 list of the richest Americans has just been released, and Buffett's friend Bill Gates, the former Microsoft Chairman, is at the top of the rankings, with an estimated net worth of $57 billion.

That tops Buffett's $50 billion, although he's still number two out of 400 on the Forbes list of the nation's super-rich.

The magazine notes that Berkshire Hathaway shares are down 15 percent since Buffett was named the world's richest billionaire six months ago with an estimated $62 billion. They had soared to an all-time high late last year, lifting him into first place.

Since today's list includes only Americans, we're not really sure if Gates is also the world's richest man. We only know that Buffett isn't.

It's possible, of course, that he could leapfrog over Gates when Forbes releases its annual ranking of global billionaires early next year. And don't count out Mexico's Carlos Slim, who is well-positioned to move into the top slot from number two last March.

Meanwhile, Buffett may have to tighten up on his expenditures a bit. Six months ago, he joked with us that he had passed Gates on the wealth list because he "spends less."